15 lines
3.4 KiB
XML
15 lines
3.4 KiB
XML
<?xml version="1.0"?>
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<clause number="14.6.6" title="Cast expressions">
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<paragraph>A <non_terminal where="14.6.6">cast-expression</non_terminal> is used to explicitly convert an expression to a given type. <grammar_production><name><non_terminal where="14.6.6">cast-expression</non_terminal></name> : <rhs><terminal>(</terminal><non_terminal where="11">type</non_terminal><terminal>)</terminal><non_terminal where="14.6">unary-expression</non_terminal></rhs></grammar_production></paragraph>
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<paragraph>A <non_terminal where="14.6.6">cast-expression</non_terminal> of the form (T)E, where T is a type and E is a <non_terminal where="14.6">unary-expression</non_terminal>, performs an explicit conversion (<hyperlink>13.2</hyperlink>) of the value of E to type T. If no explicit conversion exists from the type of E to T, a compile-time error occurs. Otherwise, the result is the value produced by the explicit conversion. The result is always classified as a value, even if E denotes a variable. </paragraph>
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<paragraph>The grammar for a <non_terminal where="14.6.6">cast-expression</non_terminal> leads to certain syntactic ambiguities. For example, the expression (x)-y could either be interpreted as a <non_terminal where="14.6.6">cast-expression</non_terminal> (a cast of -y to type x) or as an <non_terminal where="14.7">additive-expression</non_terminal> combined with a <non_terminal where="14.5.3">parenthesized-expression</non_terminal> (which computes the value x -y). </paragraph>
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<paragraph>To resolve <non_terminal where="14.6.6">cast-expression</non_terminal> ambiguities, the following rule exists: A sequence of one or more tokens (<hyperlink>9.4</hyperlink>) enclosed in parentheses is considered the start of a <non_terminal where="14.6.6">cast-expression</non_terminal> only if at least one of the following are true: <list><list_item> The sequence of tokens is correct grammar for a type, but not for an expression. </list_item><list_item> The sequence of tokens is correct grammar for a type, and the token immediately following the closing parentheses is the token "~", the token "!", the token "(", an identifier (<hyperlink>9.4.1</hyperlink>), a literal (<hyperlink>9.4.4</hyperlink>), or any keyword (<hyperlink>9.4.3</hyperlink>) except as and is. </list_item></list></paragraph>
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<paragraph>
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<note>[Note: The above rule means that only if the construct is unambiguously a <non_terminal where="14.6.6">cast-expression</non_terminal> is it considered a <non_terminal where="14.6.6">cast-expression</non_terminal>. end note]</note>
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</paragraph>
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<paragraph>The term "correct grammar" above means only that the sequence of tokens must conform to the particular grammatical production. It specifically does not consider the actual meaning of any constituent identifiers. For example, if x and y are identifiers, then x.y is correct grammar for a type, even if x.y doesn't actually denote a type. </paragraph>
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<paragraph>
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<note>[Note: From the disambiguation rule, it follows that, if x and y are identifiers, (x)y, (x)(y), and (x)(-y) are <non_terminal where="14.6.6">cast-expression</non_terminal>s, but (x)-y is not, even if x identifies a type. However, if x is a keyword that identifies a predefined type (such as <keyword>int</keyword>), then all four forms are <non_terminal where="14.6.6">cast-expression</non_terminal>s (because such a keyword could not possibly be an expression by itself). end note]</note>
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</paragraph>
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</clause>
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